The present invention relates to silicon nitride based ceramics and their use, particularly as cutting tools.
In the past, it has been taught by U.S. Pat. No. 4,652,276 that beta silicon nitride compositions useful to machine cast iron must contain both yttrium oxide (yttria) and magnesium oxide (magnesia) in the range of 5 to 20 weight percent (w/o), total, to obtain long tool life (i.e. improved wear resistance) and improved chipping resistance in the machining of nodular cast iron.
Y.sub.2 O.sub.3 and MgO are added in the amounts indicated to produce a glassy intergranular phase during sintering in an amount necessary to the achievement of the proper densification of the ceramic and improved metal cutting performance.
It was found that compositions composed of 98 w/o Si.sub.3 N.sub.4 -1 w/o MgO-1 w/o Y.sub.2 O.sub.3 have poor chipping resistance and poor wear resistance compared to the compositions in accordance with U.S. Pat. No. 4,652,276 (see col. 4, tables I and II).
There, however, remains a need for more advanced silicon nitride ceramics and cutting tools made therefrom which have improved properties and cutting performance, but can also be densified by economical densification methods.